Optical amplification in coherent optical frequency modulated continuous wave reflectometry

ABSTRACT

The present invention relates to an apparatus for optical coherence reflectometry, in particular for optical coherence tomography, wherein the apparatus for optical coherence reflectometry comprises a wavelength scanning laser source for providing a light signal, and splitting means for dividing said light signal into a sample light field and a reference light field, wherein the sample light field is directed to the sample being measured, and the light reflected from the sample is amplified without correspondingly amplifying the light reflected in the reference light field. Thereby, it is possible to direct substantially all light energy from the first reflected light field to the detectors, and to obtain fully the utilisation of the amplification of the first reflected light field. The optical amplifier inserted in the sample reflected light field is different from the source so that the effect of the light source may be regulated independent of the degree of amplification. In particular when using the apparatus in coherent optical FMCW reflectometry certain safety regulations for the power density towards the sample has to be observed to reduce the risk of damages to the sample under examination, such as biological tissue. The apparatus may be used for a variety of purposes, in particular for obtaining optical biopsies of transparent as well as non-transparent tissues.

[0001] The present invention relates to an apparatus for opticalcoherence reflectometry, in particular for optical coherence tomography.

BACKGROUND

[0002] Optical low-coherence reflectometry (OLCR) is used for examplefor analyzing inhomogeneities in optical waveguides and optical devices.In this method light is transmitted down the optical fibre and lightresulting from the interaction with an inhomogeneity in the opticalfibre is back-scattered. The light is split into two arms, a sample armand a reference arm. When the optical pathlength in the sample armmatches time delay in the reference arm coherent interference occurs andthe distance the light has travelled in the sample arm may bedetermined.

[0003] Most known devices, use broadband light sources eg.superluminescent diodes, with a short coherence time, and they need ascanning mirror to record the depth resolved backscattered signal. Inother systems a tunable laser is used as the light source, whereby,instead of moving the mirror, the wavelength of the laser can be variedto record the backscattered signal. This principle is discussed inHaberland, U.H.P. et al., “Chirp Optical Coherence Tomography of LayeredScattering Media” as well as in U.S. Pat. No. 5,956,355 (Swanson etal.). The method is often referred to as coherent optical frequencymodulated continuous wave (FMCW) reflectometry.

[0004] OLCR can be extended through the use of polarized light. Thelight field towards the reference and sample is then polarized. Aftercombining the light field reflected from the reference and the sample,the combined light field is split up again into two new light fieldswith perpendicular polarization states. Through this method thebirefringent properties of the sample can be investigated in addition tothe information obtainable with ordinary OLCR adding to the systemsability to discriminate between certain types of materials within thesample. This method also applies to OCT often referred to aspolarization sensitive OCT (PS-OCT), as well as coherent optical FMCWreflectometry.

[0005] Optical low-coherence reflectometry is also used in the imagingof 2-dimensional and 3 dimensional structures, eg. biological tissues,in this respect often referred to as optical coherence tomography (OCT).OCT can be used to perform high-resolution cross-sectional in vivo andin situ imaging of microstructures, such as in transparent as well asnon-transparent biological tissue or other absorbing and/or random mediain generel. There are a number of applications for OCT, such asnoninvasive medical diagnostic tests also called optical biopsies. Forexample cancer tissue and healthy tissue can be distinguished by meansof different optical properties. Coherent optical FMCW reflectometryalso applies to the above-mentioned cases.

[0006] In order to optimize optical low-coherence reflectometrymeasurements and imaging various suggestions to increase signal-to-noiseratio (SNR) have been discussed in the art.

[0007] U.S. Pat. No. 5,291,267 (Sorin et al.) discloses opticalreflectometry for analyzing inhomogeneities in optical fibres. In U.S.Pat. No. 5,291,267 amplification of the light reflected from the opticalfibre is conducted. In particular U.S. Pat. No. 5,291,267 suggests touse the light source as an amplifier in order to save costs.

[0008] WO 99/46557 (Optical Biopsies Technologies) discusses SNR in asystem wherein a reference beam is routed into a long arm of aninterferometer by a polarizing beamsplitter. In general the referencesuggest to include an attenuator in the reference arm to increase SNR.In a balanced setup the reference on the other hand suggests to increasethe power of the reference arm in order to increase SNR.

[0009] In “Unbalanced versus balanced operation in an optical coherencetomography system” Podoleanu, A. G., Vol. 39, No. 1, Applied optics,discussed various methods of increasing SNR in unbalanced and balancedsystems, respectively. Reduction of power in the reference arm wassuggested as well as reduction of fibre end reflections to increase theSNR.

[0010] Optical low-coherence tomography reflectometry and coherentoptical FMCW reflectometry obtain the same information about the samplebeing investigated, and, in this respect, they may be consideredsimilar.

[0011] The present invention relates to an optimisation coherent opticalFMCW reflectometry whereby an increase of the SNR is obtained leading toa better result of the measurements, in particular in relation topenetration depth of the system, so that the penetration depthincreases, when the SNR increases.

SUMMARY OF THE INVENTION

[0012] Thus, the present invention relates to an apparatus for opticalcoherence reflectometry comprising

[0013] a wavelength scanning laser source for providing a light signal

[0014] splitting means for dividing said light signal into a first lightfield and a second light field,

[0015] means for directing the first light field to a sample, and meansfor directing a first reflected light field from the sample, wherein anoptical amplifier is inserted in the first reflected light field, saidoptical amplifier being different from the light source, and means fordirecting the amplified first reflected light field to a combiningmeans, so that the amplified first reflected light field is directed tothe combining means through another route than a route through thesplitting means for dividing the light signal,

[0016] means for directing the second light field to the combiningmeans,

[0017] combining means for receiving said amplified first reflectedlight field and said second light field to generate a combined lightsignal, and

[0018] at least one detecting means for detecting the combined lightsignal and outputting detection signals.

[0019] In the present context the term “optical coherence reflectometry”is used in its normal meaning, and in particular the term means opticalcoherence FMCW reflectometry.

[0020] Furthermore, the term “wavelength scanning laser source” means afrequency-tuned laser having a tunable longitudinal cavity mode and acenter tunable wavelength, for example as described in U.S. Pat. No.5,956,355.

[0021] The present apparatus offers a better signal-to-noise ratio (SNR)whereby an increase of the maximal penetration depth is obtained.Thereby, the apparatus according to the present invention is especiallyuseful for obtaining optical biopsies of transparent as well asnon-transparent tissues.

[0022] In particular a combination of the arrangement of amplificationdiscussed above and reduction of fibre end reflections increases thesignal-to-noise ratio leading to an improved system.

[0023] The term “sample path” or “sample arm” is used to define theroute travelled by the light from the light source to the sample andreflected from the sample to the combining means. In the present contextthe light field and routes relating to the sample arm is denoted thefirst light field and the first light route, respectively.

[0024] Correspondingly the term “reference path” or “reference arm” isused to define the route travelled by the light from the light source tothe combining means. In the present context the light field and routesrelating to the reference arm is denoted the second light field and thesecond light route, respectively. It is often convenient to be able toalter the optical length of the second light route. This may beaccomplished by insertion of reflection means where the position ofthese may be scanned or by using a so-called fiber-stretcher well knownin the art.

[0025] In another aspect the present invention relates to a method forproviding a result of a sample comprising

[0026] establishing a wavelength scanning laser source for providing alight signal,

[0027] splitting said light signal into a first light field and a secondlight field,

[0028] directing the first light field to a sample, and the second lightfield to a reference path,

[0029] receiving the first reflected light field from the sample,optically amplifying the first reflected light field, and directing thefirst reflected light field in a combining means,

[0030] receiving the second light field,

[0031] combining said amplified first reflected light field and saidsecond light field to generate a combined light signal,

[0032] detecting the combined light signal obtaining detection signals,and

[0033] processing the detection signals obtaining the result of thesample.

[0034] In the present context the term “result of the sample” may referin coherent optical FMCW reflectometry to the image of the sampleobtained. When using the present invention in coherent optical FMCWreflectometry in optical fibres used for example in the communicationtechnology the result relates to the signal obtained, such as a signalrelating to the distance to an inhomogeneity in the device under test.

DRAWINGS

[0035]FIG. 1 shows an unbalanced conventional coherent optical FMCWreflectometry system according to prior art, wherein an attenuator hasbeen inserted in the reference arm.

[0036]FIG. 2 shows a balanced apparatus according to the invention,wherein the amplified first reflected light field is directed to thecombining means through another route than a route through the splittingmeans. A y-coupler is inserted in the sample arm to receive thereflected light field from the sample.

[0037]FIG. 3 shows the balanced detection means of FIG. 2 in detail.

[0038]FIG. 4 shows a balanced apparatus as in FIG. 2 wherein an opticalcirculator has been inserted instead of the beam splitting means in thesample.

[0039]FIG. 5 shows a balanced system chosen as reference system. Thesystem is similar to the system shown in FIG. (2) except for omittion ofthe optical amplifier and the y-coupler in the sample part. They-coupler is omitted since it is no longer necessary for the light tofollow a different path to and from the sample.

[0040]FIG. 6 shows the optimum splitter ratio for the system shown inFIG. (2) investigated in the absence of an optical amplifier, i.e. theamplification factor is set to 1 and the optical noise added from theamplifier is set to zero. The SNR of the system is compared to thereference system, where both systems are used in the uncoated case.

[0041]FIG. 7 shows the optimum splitter ratio for the system shown inFIG. (2) investigated in the absence of an optical amplifier, i.e. theamplification factor is set to 1 and the optical noise added from theamplifier is set to zero. The SNR of the system is compared to thereference system, where both systems are used in the coated case.

[0042]FIG. 8 shows the effect of including an optical amplifier on thenovel system shown in FIG. (2). The SNR of the novel system is comparedto that of the reference system FIG. (5), i.e.SNR_(novel)/SNR_(reference), where both systems are used in the uncoatedcase.

[0043]FIG. 9 shows the effect of including an optical amplifier on thenovel system shown in FIG. (2). The SNR of the novel system is comparedto that of the reference system FIG. (5), i.e.SNR_(novel)/SNR_(reference), where both systems are used in the coatedcase.

[0044]FIG. 10 shows the optimum splitting ratio for the novel systemshown in FIG. (2), with the optical amplifier set at a fixedamplification factor of 20 dB. The SNR of the novel system is comparedto the reference system FIG. (5), where both systems are used in theuncoated case.

[0045]FIG. 11 shows the optimum splitting ratio for the novel systemshown in FIG. (2), with the optical amplifier set at a fixedamplification factor of 20 dB. The SNR of the novel system is comparedto the reference system FIG. (5), where both systems are used in thecoated case.

[0046]FIG. 12 shows the relative SNR shown as function of the thermalnoise for the system shown in FIG. (4), where the splitting ratio is setto the optimum setting and r_(und) is taken as the coated case.

[0047]FIG. 13 shows a balanced apparatus according to the invention,wherein only the first reflected light field is amplified by the opticalamplifier and hereafter directed to the combining means.

[0048]FIG. 14 shows the optimum splitter ratio for the system shown inFIG. (13) investigated in the absence of an optical amplifier, i.e. theamplification factor is set to 1 and the optical noise added from theamplifier is set to zero. The SNR of the system is compared to thereference system, where both systems are used in the uncoated case.

[0049]FIG. 15 shows the optimum splitter ratio for the system shown inFIG. (13) investigated in the absence of an optical amplifier, i.e. theamplification factor is set to 1 and the optical noise added from theamplifier is set to zero. The SNR of the system is compared to thereference system, where both systems are used in the coated case

[0050]FIG. 16 shows the effect of including an optical amplifier on thenovel system shown in FIG. (13). The SNR of the novel system is comparedto that of the reference system FIG. (5), i.e.SNR_(novel)/SNR_(reference), where both systems are used in the uncoatedcase.

[0051]FIG. 17 shows the effect of including an optical amplifier on thenovel system shown in FIG. (13). The SNR of the novel system is comparedto that of the reference system FIG. (5), i.e.SNR_(novel)/SNR_(reference), where both systems are used in the coatedcase.

[0052]FIG. 18 shows the optimum splitting ratio for the novel systemshown in FIG. (13), with the optical amplifier set at a fixedamplification factor of 20 dB. The SNR of the novel system is comparedto the reference system FIG. (5), where both systems are used in theuncoated case.

[0053]FIG. 19 shows the optimum splitting ratio for the novel systemshown in FIG. (13), with the optical amplifier set at a fixedamplification factor of 20 dB. The SNR of the novel system is comparedto the reference system FIG. (5), where both systems are used in thecoated case.

[0054]FIG. 20 shows the SNR of the novel system shown in FIG. (4),relative to the optimum reference system, as a function of the splittingratio x/(1−x) in the uncoated case. The optical amplifier is set at afixed amplification factor of 20 dB. The optimum splitting ratio for theset of parameter values chosen as an example is found to be 75.21/24.79.

[0055]FIG. 21 shows the SNR of the novel system shown in FIG. (4),relative to the optimum reference system, as a function of the splittingratio x/(1−x) in the coated case. The optical amplifier is set at afixed amplification factor of 20 dB. The optimum splitting ratio for theset of parameter values chosen as an example is found to be 75;59/24.41.

DESCRIPTION OF THE DRAWINGS

[0056] In FIG. 1 an unbalanced detection scheme, not according to theinvention, is shown for comparison reasons. The optical coherence systemis denoted 1. A light source 2 provides a light signal that is directedto a splitting means 3 for dividing said light signal into a first lightfield 4 and a second light field 5. The splitting ratio in FIG. 1 is setto 50/50. The first reflected light 9 and the second light 10 iscombined by the splitter means 3 and a combined signal is directed tothe detector 8. The second light field reflected from the reflectionmeans 6 is attenuated by attenuator 7.

[0057] In FIG. 2 a detection scheme according to the invention isdepicted. The optical coherence system is denoted 1. A light source 2provides a light signal that is directed to a splitting means 3 fordividing said light signal into a first light field 4 and a second lightfield 5. The splitting ratio may be set to any suitable ratio,exemplified by the ratio x/1−x. The first reflected light field 9 isdirected back through another route than to the splitting means 3, andamplified in the optical amplifier 12, and thereafter directed to thebalanced detection means 11 comprising a combining means. The secondlight field 10 reflected from the reflection means 6 is also directed tothe balanced detection means 11. The reflection means 6 is shown as aso-called corner cube configuration.

[0058] The balanced detection means 11 is shown in detail in FIG. 3comprising a combining means 13, exemplified by a splitter having asplitting ratio of 50/50, capable of splitting the combined signal intofirst split signal 14 and second split signal 14′. The split signals 14,14′ are directed to the detectors 8, 8′ respectively. The two detectedsplit signals are subtracted to obtain an output signal. The outputsignal may be output via 15 to a printing means, a display and/or astorage means.

[0059] In FIG. 4, a refinement of the system 1 shown in FIG. 2 is shown.To avoid the reduction of the first reflected light field 9 by thesplitting means 3, an optical circulator 16 is inserted to directsubstantially all the light power in the first reflected light field 9to the optical amplifier 12.

[0060] In FIG. 13 a preferred detection scheme according to theinvention is depicted. The optical coherence system is denoted 1. Alight source 2 provides a light signal that is directed to a splittingmeans 3 for dividing said light signal into a first light field 4 and asecond light field 5. The splitting ratio may be set to any suitableratio, exemplified by the ratio x1−x. The first reflected light field 9is directed back to the splitting means 3. After the splitter, the firstreflected light filed 9 is—due to the nature of the splitterused—reduced by the factor (1−x) and directed to the optical amplifier12, and thereafter to the balanced detection means 11 comprising acombining means. The second light field 10 reflected from the reflectionmeans 6 is also directed to the balanced detection means 11. Thereflection means 6 is shown as a so-called corner cube configuration.

DETAILED DESCRIPTION

[0061] The present invention relates to an apparatus for coherentoptical FMCW reflectometry, in particular optical coherence tomography.

[0062] One important aspect of the present invention is the route of thelight field in the sample arm. The first reflected light field isamplified before being received by a combining means, said combiningmeans being capable of receiving the first reflected light field fromthe sample arm as well as the second light field from the reference arm.The amplified first reflected light field is directed to the combiningmeans through another route than a route through the splitting means fordividing the light signal from the light source into the sample arm andthe reference arm, respectively. Thereby, it is possible to directsubstantially all light energy from the first reflected light field tothe combining means, and to obtain fully the utilisation of theamplification of the first reflected light field. In other words by thepresent invention the amplified first reflected light field is directedto the combining means, so that only the reflected light field isamplified by the optical amplifier.

[0063] Another important aspect of the invention is that the opticalamplifier inserted in the first reflected light field is different fromthe light source so that the effect of the light source may be regulatedindependent of the degree of amplification. In particular when using theapparatus in coherent optical FMCW reflectometry certain safetyregulations for the power density towards the sample has to be observedto reduce the risk of damages to the sample under examination, such asbiological tissue.

[0064] Light Source

[0065] The wavelength scanning laser source provides the light signalfor use in the method and system. The wavelengths scanned are adjustedto the purpose of the analysis performed with the apparatus. Thewavelengths are mostly selected in the range from 500 nm to 2000 nm. Fornon-transparent solid tissue the wavelength is normally selected in therange from 1250 nm to 2000 nm.

[0066] For retinal examinations the wavelength is mostly selected in therange from 600 nm to 1100 nm.

[0067] Balanced/Unbalanced System

[0068] In general the system or apparatus according to the invention maybe constructed as either an unbalanced system or a balanced system. Theterms unbalanced and balanced are used in the normal meaning, ie. anunbalanced system refers to a system having one detecting means, whereasa balanced system refers to a system having two detecting means, whereineach detector receives signals from the sample arm as well as from thereference arm. In a balanced system the signals from the two detectorsare subtracted from each other in order to obtain the result.

[0069] Also a double balanced system may be used in the apparatusaccording to the invention, a double balanced system referring to asystem comprising four detecting means.

[0070] Noise

[0071] The noise in the apparatus or system according to the inventionis the total sum of noise sources in the following parts:

[0072] Optical noise, such as noise from the light source and noise fromthe optical amplifier.

[0073] Receiver noise, such as thermal fluctuations in the electronicparts and shot noise.

[0074] The optical noise from the light source is manifested as thephase noise relating to the first reflected light and phase noiserelating to the second light as well as phase noise from a mixture ofboth.

[0075] In the following calculations, a specific configuration for thebalanced detection scheme applied to low coherent reflectometry has beenchosen. However, the added benefit of introducing an optical amplifierwith respect to the signal-to-noise-ratio also applies to otherrealizations of balanced detection and double balanced detectionschemes.

[0076] For the following calculation a balanced detector system isassumed comprising of two detectors and a fiber-optic splitter as shownin FIG. 3.

[0077] The receiving device is assumed to receive two light fields, seeFIG. 3: From the reference arm the field is E_(ref)(t) having theintensity I_(ref)(t) from the sample the field is E_(sam)(t) withintensity I_(sam)(t).

[0078] Assuming that the coupler used in the balanced detector, see FIG.3, is symmetric, the field incident on each detector 1 and detector 2respectively can be written as $\begin{matrix}{{\begin{bmatrix}{E_{1}(t)} \\{E_{2}(t)}\end{bmatrix} = {{^{\quad \phi_{r}}\begin{bmatrix}a & {b\quad ^{\quad \phi}} \\{b\quad ^{\quad \phi}} & a\end{bmatrix}}\begin{bmatrix}{E_{sam}(t)} \\{E_{ref}(t)}\end{bmatrix}}},} & (1)\end{matrix}$

[0079] where φ_(r) and φ expresses phase changes due to the coupler, tthe time, j={square root}{square root over (−1)} and a and b arecoupling constants. It is known from the art that if the coupler isassumed lossless this constraint will mean that a²+b²=1 and φ=±π/2. Fora 50/50 coupler a =b=1/{square root}2. Thus for the balanced detectorthe incident fields are: $\begin{matrix}{{\begin{bmatrix}{E_{1}(t)} \\{E_{2}(t)}\end{bmatrix} = {{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & ^{\quad {\pi/2}} \\^{\quad {\pi/2}} & 1\end{bmatrix}}\begin{bmatrix}{E_{sam}(t)} \\{E_{ref}(t)}\end{bmatrix}}},} & (2)\end{matrix}$

[0080] where the common phase change φ_(r) has been assumed zero withoutloss of generality. Using this it is straight forward to calculate theelectrical current i₁ and i₂ in each detector due to a square lawdetection of the incident light power: $\begin{matrix}{\begin{bmatrix}{i_{1}(t)} \\{i_{2}(t)}\end{bmatrix} = {\frac{1}{2}{{\alpha \begin{bmatrix}{{I_{sam}(t)} + {I_{ref}(t)} + {{E_{sam}(t)}{E_{ref}^{*}(t)}^{{- }\quad {\pi/2}}} + {{E_{sam}^{*}(t)}{E_{ref}(t)}^{\quad {\pi/2}}}} \\{{I_{sam}(t)} + {I_{ref}(t)} + {{E_{sam}(t)}{E_{ref}^{*}(t)}^{\quad {\pi/2}}} + {{E_{sam}^{*}(t)}{E_{ref}(t)}^{{- }\quad {\pi/2}}}}\end{bmatrix}}.}}} & (3)\end{matrix}$

[0081] α=ηe/hν is the responsivity of the photodetectors used in abalanced system setup where e is the electron charge, h Planck'sconstant, ν the average wavelength of the light source, η the quantumefficiency of the photodetectors. Since the balanced detector detectsthe difference between the two currents, the received electrical signali(t) becomes:

i(t)=−αi(E_(sam)(t)E*_(ref)(t)−E*_(sam)(t)E _(ref)(t))  (4)

[0082] FMCW Spectrum

[0083] The signal in a FMCW system is obtained through a narrow linewidth light source where the frequency is scanned, and the resultingsignal current is Fourier-transformed to obtain the desired information.If the optical frequency is scanned linearly and the source is assumedto only exhibit phase noise, the field from the source can be written as$\begin{matrix}\begin{matrix}{{E_{source}\left( t^{\prime} \right)} = {E_{0}{\exp \left\lbrack {j\quad \left( {{{\omega \left( t^{\prime} \right)}t^{\prime}} + \phi_{t^{\prime}}} \right)} \right\rbrack}}} \\{{= {E_{0}{\exp \left\lbrack {j\quad \left( {{\omega_{0}t^{\prime}} + {\pi \quad \gamma \quad t^{\prime 2}} + \phi_{t^{\prime}}} \right)} \right\rbrack}}},}\end{matrix} & (5)\end{matrix}$

[0084] where ω(t) is the angular frequency as a function of time, ω₀ theangular frequency offset, γ the frequency scan speed E₀ is the amplitudeand φ₁ is the random fluctuation phase at time t′. The reference fieldand the field from the sample arm originate from the same source and canbe written as:

E _(ref)(t)=E _(r) exp└j(ω₀ t+πγt ²+φ_(t))┘  (6) $\begin{matrix}{{{E_{sam}(t)} = {\int_{- \infty}^{\infty}{\sqrt{r\left( \tau_{0} \right)}E_{s}{\exp \left\lbrack {j\quad \left( {{\omega_{0}\left( {t + \tau_{0}} \right)} + {\pi \quad {\gamma \left( {t + \tau_{0}} \right)}^{2}} + \phi_{t + \tau_{0}}} \right)} \right\rbrack}{\tau_{0}}}}},} & (7)\end{matrix}$

[0085] where τ₀ is the time delay due to difference in optical pathlength between the sample and reference arm, E_(r) and E_(s) therespective amplitudes and r(τ₀) is a function describing the intensityreflectivity profile of the sample arm. This reflectivity profileincludes the reflectivity profile of the sample and any undesiredreflections in the sample arm e.g. from lenses, fiber ends, etc. Next,we investigate the received signal due to a single reflection in thesample arm i.e. the case where r(τ₀)=δ(τ₀), where δ is the Dirac deltafunction. Adapting the calculation of the spectrum of the receivedphotocurrent given by S. Venkatesh and W. Sorin (“Phase NoiseConsiderations in Coherent Optical FMCW Reflectometry”, J. of Lightw.Tech., VOL 11, No. 10, 1993) to a balanced system the single sidedspectrum of the signal current is found to be $\begin{matrix}{\frac{S(f)}{\alpha^{2}E_{r}^{2}E_{s}^{2}} = {{2\quad {\exp \left\lbrack {- \frac{f}{\tau_{c}\gamma}} \right\rbrack}{\delta \left( {f - f_{b}} \right)}} + {\frac{4\tau_{c}}{1 + \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)^{2}}\left( {1 - {{\exp \left\lbrack {- \frac{\tau_{0}}{\tau_{c}}} \right\rbrack}\left( {{\cos \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)} + \frac{\sin \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)}{2{{\pi\tau}_{c}\left( {f - f_{b}} \right)}}} \right)}} \right)}}} & (8)\end{matrix}$

[0086] where f_(b)=γτ₀ is the beat frequency due to path lengthdifference between the reference and sample fields, τ_(c)=½πΔγ is thecoherence time of the light source and Δγ is the full width half max(FWHM) of the line width of the source.

[0087] The first term of Eq.(8) is the signal due to the reflection inthe sample arm, and the second term is a broadband noise contributiondue to the phase noise of the light source. Inspecting Eq.(4) it isclear that since there is no mixing terms of the sample field withitself the current resulting from multiple reflections in the samplewill be a superposition of the current resulting from each reflectionhad it been alone. Thus, the single sided spectrum of the signal currentis found to be $\begin{matrix}{\frac{S(f)}{\alpha^{2}E_{r}^{2}E_{s}^{2}} = {{2\quad {\exp \left\lbrack {- \frac{f}{\tau_{c}\gamma}} \right\rbrack}{r\left( \frac{f}{\gamma} \right)}} + {\int_{- \infty}^{\infty}{\frac{4\tau_{c}{r\left( \tau_{0} \right)}}{1 + \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)^{2}}\left( {1 - {{\exp \left\lbrack {- \frac{\tau_{0}}{\tau_{c}}} \right\rbrack}\left( {{\cos \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)} + \frac{\sin \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)}{2{{\pi\tau}_{c}\left( {f - f_{b}} \right)}}} \right)}} \right){{\tau_{0}}.}}}}} & (9)\end{matrix}$

[0088] Optical Amplification

[0089] In this calculation an optical amplifier is modeled to amplifythe incoming light and add optical noise due to intrinsic amplifiernoise. Hence, the light intensity emitted from an optical amplifier isgiven by

I _(out)

=G

I _(in)

+

I _(noise)

,  (10)

[0090] where G is the amplification factor, I_(in) the intensity of theincident light, and I_(noise) the intensity due to intrinsic amplifiernoise.

[0091] The term I_(noise) added by the optical amplifier contributes tothe system noise in two ways. Firstly, through a mixing term with thereference field in the art known as signal-local oscillator noise, andsecondly through adding to the shot noise. According to the art (seee.g. N. A. Olsson, “Lightwave Systems with optical Amplifiers”, AT&TBell Laboratories, J. of Lightwave Tech., Vol. 7, No. 7, 1983) theoptical noise power emitted by the optical amplifier is given by

I _(noise)

=N _(sp)(G−1)hvB ₀,  (11)

[0092] where N_(sp) is the spontaneous emission factor, ν the centerfrequency of the optical bandwidth of the amplifier B_(o), and hPlanck's constant. The bandwidth B_(o), should be chosen to span overthe wavelengths scanned by the light source.

[0093] Noise Contributions

[0094] The noise contributions are all expressed as the received noisepower after electrical subtraction of the two signals received by eachphotodetector per unit bandwidth.

[0095] Phase Noise

[0096] In Eq.(9) the second term represents the noise contribution dueto phase noise. To estimate this contribution a realistic reflectivityprofile, r(τ₀), of the sample arm is constructed where the sample is,without loss of generality, chosen to be a highly scattering tissue.This profile consists of three elements: an undesired reflection fromthe optics before the sample e.g. a fiber end or a lens, the desiredreflections inside the sample and a distributed reflectivityexponentially decreasing due to backscattering within the highlyscattering tissue. r(τ₀) is written as $\begin{matrix}{{{r\left( \tau_{0} \right)} = {{r_{und}{\delta \left( {\tau_{0} - \tau_{und}} \right)}} + {\sum\limits_{i}^{\quad}{r_{i}{\exp \left\lbrack {{- 2}\mu \quad \tau_{i}} \right\rbrack}{\delta \left( {\tau_{0} - \tau_{i}} \right)}}} + {r_{b}{\exp \left\lbrack {{- 2}{\mu\tau}_{0}} \right\rbrack}}}},} & (12)\end{matrix}$

[0097] where μ is the damping coefficient of the medium, r_(und) is thereflectivity of the undesired reflection, r_(i) is the reflectivity ofthe respective discrete reflection, and r_(b) is the fraction of thelight lost due to damping which goes to backscattering. The noisecontribution from phase noise is thus

Δi _(phase) ²

=α² E _(r) ² E _(s) ² S _(n)(ƒ),  (13)

[0098] where S_(n)(f) is given by $\begin{matrix}{{S_{n}(f)} = {\int_{- \infty}^{\infty}{\frac{4\tau_{c}{r\left( \tau_{0} \right)}}{1 + \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)^{2}}\left( {1 - {{\exp \left\lbrack {- \frac{\tau_{0}}{\tau_{c}}} \right\rbrack}\left( {{\cos \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)} + \frac{\sin \left( {2\pi {\tau_{0}}\left( {f - f_{b}} \right)} \right)}{2{{\pi\tau}_{c}\left( {f - f_{b}} \right)}}} \right)}} \right){{\tau_{0}}.}}}} & (14)\end{matrix}$

[0099] Amplifier Noise

[0100] Since the system utilizes a balanced detection scheme the noiseemitted by the amplifier, I_(noise), can only contribute to the noise ofthe system through mixing with the reference field. This contribution isgiven by (see e.g. N. A. Olsson, “Lightwave Systems with opticalAmplifiers”, AT\&T Bell Laboratories, J. of Lightwave Tech., Vol. 7, No.7, 1983):

Δi _(noise) ²

=4α²

hν

I _(ref)

N _(sp)(G−1).  (15)

[0101] Shot Noise

[0102] It is common knowledge within the art that the so-calledshot-noise due to the particle nature of the photon-to-electronconversion in the photodetectors is given by $\begin{matrix}\begin{matrix}{{\langle{\Delta \quad i_{shot}^{2}}\rangle} = {2e\quad \alpha {\langle\quad I_{total}\rangle}}} \\{= {2e\quad {\alpha \left( {{G{\langle I_{sam}\rangle}} + {\langle I_{ref}\rangle} + {\langle I_{noise}\rangle}} \right)}}}\end{matrix} & (16)\end{matrix}$

[0103] where I_(total) is the total light intensity entering thesplitter of the balanced detector (see FIG. (3)).

[0104] Receiver Noise

[0105] The photodetectors also have an inherent noise contribution,which is independent of the incident light power. There are twocontributions to this noise: Thermal noise in the electrical circuit ofthe detectors and shot-noise due to dark current in photodetector. Thereceiver noise is: $\begin{matrix}{{{\langle{\Delta \quad i_{reciever}^{2}}\rangle} = {{\langle{\Delta \quad i_{thermal}^{2}}\rangle} = {{\langle{\Delta \quad i_{dark}^{2}}\rangle} = {2\left( {{\frac{4k_{B}T}{R}F_{n}} + {2e{\langle{\Delta \quad i_{dark}^{2}}\rangle}}} \right)}}}},} & (17)\end{matrix}$

[0106] where kb is Boltzman's constant, R the load resistance of each ofthe detectors, T the temperature, F_(n) the noise figure of theelectrical circuit of the detectors normally dominated by thepreamplifier, (i_(dark)) the dark current in each detector and thefactor of 2 is due to having two independent detectors. However, thisreceiver noise, independent of the incident light, is often moreconveniently measured experimentally.

[0107] Thermal fluctuations in the electronic parts are independent ofthe amount of light used, and furthermore, the thermal fluctuations maybe reduced by cooling of the detectors, and also by optimising theconstruction.

[0108] Shot noise relates to the particle nature of the light. The shotnoise is proportional to the amount of light received.

[0109] When inserting an optical amplifier in the apparatus it isinherent that in addition to the amplification of the signal desired theoptical noise will inevitably also be amplified.

[0110] Received Signal Power

[0111] From Eq.(8) is can be seen that the signal power due to a singlereflection with the time delay to the reference τ₀ is given by$\begin{matrix}{{{\langle{\Delta \quad i_{signal}^{2}}\rangle} = {{\exp \left\lbrack {{- \tau_{0}}/\tau_{c}} \right\rbrack}\alpha^{2}E_{r}^{2}E_{s}^{2}{\int_{\tau_{0}^{-}}^{\tau_{0}^{+}}{{r\left( \tau_{0} \right)}{\tau_{0}}}}}},} & (18)\end{matrix}$

[0112] when one measures the signal over a finite bandwidth: γ(τ₀ ⁺−τ₀⁻).

[0113] Signal-to-Noise Ratio

[0114] Using the above equations it is straightforward to derive thesignal-to-noise ratio (SNR): $\begin{matrix}{{SNR} = \frac{\langle\quad i_{signal}^{2}\rangle}{{B\left( {{\langle{\Delta \quad i_{receiver}^{2}}\rangle} + {\langle{\Delta \quad i_{shot}^{2}}\rangle} + {\langle{\Delta \quad i_{amp}^{2}}\rangle} + {\langle{\Delta \quad i_{phase}^{2}}\rangle}} \right)},}} & (19)\end{matrix}$

[0115] where B is the bandwidth over which a signal is detected i.e. theresolution of the Fourier-transform of the signal current. However, asthe light source is scanned over a finite interval, a time window isimposed upon the signal current leading to a convolution of the signalspectrum with the Fourier-transform of the this time window. The widthof this transform determines the resolution of the system together withthe bandwidth of the detection system, which determines the smallestfrequency increment detectable. Notice, that this bandwidth is decidedby the scanning of the light source and the electrical detection system,so in a comparison of the performance of different systems, which usesthe same light source and electrical detector system, the bandwidthinvolved will be a common factor, and is thus ignored for the rest ofthis analysis.

[0116] For a given light source, sample and electrical detector systemthe field amplitudes E_(r) and E_(s) must be found according to thechosen system configuration. For this analysis the system shown inFIG.(2) has been chosen. From inspection it is straightforward to seethat if the light source emits the light intensity I_(source) is then

E _(r)={square root}{square root over ((1−x)βz,900 I _(source)

)}  (20)

[0117] and $\begin{matrix}{E_{s} = \sqrt{\frac{1}{4} \times {\langle I_{source}\rangle}}} & (21)\end{matrix}$

[0118] where x is the coupling ratio towards the sample of the firstcoupler from the source, and β is a factor describing the loss of powerdue to an inserted device for altering the optical path length of thesecond light field, such as a mirror, retro-reflector orfiber-stretcher. For simplicity, and without loss of generality, thefactor β is set to unity. This leads to the light intensities

I _(ref)

=(1−x)β

I _(source)

  (22)

[0119] and $\begin{matrix}{{\langle I_{sam}\rangle} = {\frac{1}{4} \times {\langle I_{source}\rangle}{\int_{- \infty}^{\infty}{{r^{2}\left( \tau_{0} \right)}{\tau_{0}}}}}} & (23)\end{matrix}$

[0120] Combining Eq.(20), Eq. (21), Eq. (22), and Eq.(23) with Eq. (13),Eq.(15), Eq. (16), Eq. (17) and Eq.(18) it is straight forward tocalculate Eq.(19) which can be used for comparison of system performancebetween different configurations.

[0121] In analogy with this derivation, the SNR can be derived for awide variety of low-coherent reflectometer systems with balanceddetection and thus the performance of such systems can be easilycompared.

[0122] Splitting Means

[0123] The general principle of coherent optical FMCW reflectometry isthat distance travelled by the light in the sample arm is correlated tothe distance travelled by the light in reference arm.

[0124] The light is emitted from a light source as discussed above anddivided into a first light field and a second light field by a splittingmeans. The splitting means may be any means suitable for splitting alight signal into two light fields. The splitting means may be selectedfrom any suitable splitting means, such as a bulk optic splitting means,a fibre optic splitting means, a holographic optical element or adiffractive optical element.

[0125] In one embodiment the apparatus according to the inventioncomprises a splitting means capable of dividing the light signal intothe sample arm and the reference arm with a splitting ratio of thesplitting means being substantially 50%/50%.

[0126] However the present inventors have found due to the location ofthe amplifier as well as the route of the first reflected light fieldthat a further increase in SNR may be obtained when using a changeablesplitting ratio, so that from 1% to 99% of the light energy from thelight source is directed to the sample arm. It is preferred that morethan 50% of the light energy is directed to the sample, such as from 50%to 99% of the light energy from the light source is directed to thesample arm, such as from 55% to 90% of the light energy from the lightsource is directed to the sample arm, such as from 60% to 85% of thelight energy from the light source is directed to the sample arm, suchas from 65% to 85% of the light energy from the light source is directedto the sample arm.

[0127] In another embodiment it is more preferred that from 1% to 60% ofthe light energy from the light source is directed to the sample arm,such as from 20% to 55% of the light energy from the light source isdirected to the sample arm, such as from 30% to 50% of the light energyfrom the light source is directed to the sample arm, such as from 40% to50% of the light energy from the light source is directed to the samplearm.

[0128] Sample Arm—First Light Field Route

[0129] The apparatus according to the invention comprises means fordirecting the first light field to the sample. In a preferred embodimentat least a part of the means for directing the first light field to thesample comprises an optical fibre, so that the means in total comprisesan optical fibre and an optical system. An optical system may beincluded for focusing the first light field to the sample. The opticalsystem for example being one or more lenses.

[0130] It is preferred that the first light field is directed to thesample without being amplified. Thereby the intensity of the first lightfield onto the sample is exclusively determined by the light source.This leads to a better control of the light intensity in the sample arm,since the light directed to the sample conforms to the practical limitsfor sample light, such as an upper limit for the intensity to avoiddamages to the sample, and the light reflected from the sample may beamplified to the degree necessary for the SNR to be suitably increased.Thus, the amplifier is preferably located in a part of the sample arm bywhich only the reflected light is travelling. This may be accomplishedby inserting a splitting means or a circulator to receive the reflectedfirst light field from the sample, or by inserting the optical amplifierafter the first reflected light field has passed the splitting meansused to split the light into the first and second light fields.

[0131] In a preferred embodiment a circulator is inserted wherebysubstantially all light energy reflected from the sample is directed asthe first reflected light field to the optical amplifier.

[0132] The term light field as used herein means light field as normallyused for the light in optical fibres, but does also include a light beamas normally used in bulk systems and in the optical system.

[0133] Scanning Head

[0134] The sample is scanned by means known in the art, such asgalvanometer scanners, polygon mirrors, resonant scanners, a scanninghead.

[0135] Amplifier

[0136] Any optical amplifier suitable for amplifying the reflected firstlight field may be interposed in the light route from the sample to thecombining means. The amplifier may thus be a semiconductor, a resonantamplifier or a fibre and/or Raman amplifier. The amplification factormay be in the range from 1.5 to 1,000,000 times, such as from 20 to500,000 times, for example from 20 to 100,000 times, such as from 20 to50,000 times, such as from 20 to 10,000 times, such as from 20 to 1000times, such as from 20 to 100 times.

[0137] Reference Arm Second Light Route

[0138] The apparatus according to the invention also comprises means fordirecting the second light field to the combining means. In a preferredembodiment a device is included so that the optical path length of thesecond light route may be altered. In a preferred embodiment hereof atleast a part of the directing means is comprised of an optical fiber andan optical fiber stretcher. In another preferred embodiment the deviceis a reflecting means such as a mirror setup. In this embodiment atleast a part of the means for directing the second light field to thereflecting means comprises an optical fibre, so that the directing meansin total comprises an optical fibre and an optical system. The opticalsystem may be used for directing the second light field to thereflecting means, such as any kind of lenses, gratings etc. known to theperson skilled in the art.

[0139] Attenuation of the reflected second light field may be usefulwhen using an unbalanced system, whereas attenuation of the referencearm does not add anything further to the SNR in a balanced system.

[0140] In a preferred embodiment the reflected second light field doesnot pass any splitting means for dividing the light signal whentravelling towards the combining means. It is an advantage to maintainas much as possible of the second light field on the route to thecombining means. This may be accomplished by directing the second lightfield from the splitting means to the combining means in an opticalfiber and if it is desired to alter the optical path length of thesecond light route to modulate the properties of the fiber. This may bedone through a fiber stretcher to modulate the physical length of thefiber or by e.g. applying heat to alter the refractive index of thefiber. If a reflecting means is applied to alter the optical path lengththe light power may substantially be preserved by inserting a circulatorto receive the second light field from the reflection means to directthe second light field directly to the combining means.

[0141] In a preferred embodiment a circulator is inserted to receive thesecond light field whereby substantially all light energy reflected fromthe reflecting means is directed as the second light field to thecombining means.

[0142] In another preferred embodiment a fiber stretcher is inserted asdescribed above.

[0143] The reflecting means may be any means suitable for reflecting thelight in the reference arm. The reflecting means may be a mirror oranother structure having reflective properties.

[0144] Combining Means

[0145] The combining means is any suitable means capable of receivingtwo light fields and combining the light fields into at least one lightsignal. In a preferred embodiment the combining means is a coupler.

[0146] In an unbalanced system the combining means may be identical tothe detecting means.

[0147] Detecting Means

[0148] The system comprises conventional detecting means. The detectingmeans is essentially a photodetector chosen accordingly to match thesource wavelength, a combination of photodetectors arranged to make up abalanced scheme, or a combination of photodetectors arranged to make upa double-balanced scheme. Furthermore, the detecting means may be alinear array of photodetectors without or combined with a dispersiveelement arranged so that the array provides depth and spectralinformation. The detecting means may also be a linear charge-coupleddevice (CCD) array without or combined with a dispersive elementarranged so that the array provides depth and spectral information.

[0149] Finally, the detecting means may be a two-dimensional array ofphotodetectors without or combined with a dispersive element arranged sothat the array provides depth and spectral information. The detectingmeans may also be a two-dimensional CCD array without or combined with adispersive element arranged so that the array provides depth andspectral information.

[0150] For example, the dispersive element may be a diffraction grating(reflection or transmission), a prism or a combination of prisms.

[0151] End Reflections

[0152] In a preferred embodiment the SNR is further increased byreducing non-sample reflections, such as the fibre end reflections inthe sample arm. By reducing the non-sample reflections in combinationwith amplification of the first light field an increase of the relativeSNR is increased up to for example about 10 dB, such as up to about 15dB, for example up to about 20 dB. It has been shown that theamplification of the light field in the sample arm is improvedadditionally when reducing reflections.

[0153] The end reflections may be reduced by anti-reflex coating thefibre ends of the fibres in one or both of the arms.

[0154] Also the fibre ends may be cleaved at an angle to reducereflections, such an angle being at least 5 degrees, such as preferablyat least 7 degrees.

[0155] The anti-reflex coating and the cleaving of the fibre ends may beused as alternatives or in combination.

[0156] Processing/Displaying

[0157] The result obtained may be further processed to obtain relevantinformation based on the detection signal relating to thedistance/coherence. In one embodiment the detection signal is sent to acomputer for analysis. Depending on the object scanned, the computer mayprovide an image relating to for example the tissue scanned.

[0158] In relation to detection of inhomogeneities in for exampleoptical waveguides, the computer may provide information relating to thedistance to the inhomogeity and for example also an image of theinhomogeneity.

[0159] The result may be sent from the computer to a display and/or aprinter and/or stored in a storage means.

[0160] Penetration Depth

[0161] The parameters that govern coherent optical FMCW reflectometryperformance are longitudinal and transverse resolution, dynamic range,measurement speed, and the centre wavelength of the light source.

[0162] The depth to which an illumination field of light penetrateswithin turbid media, such biological tissue or the like, is determinedby the amount of scattering and absorption present in the media.

[0163] In tissue scattering diminishes rapidly with increasingwavelength throughout the visible and infrared wavelength regions.Absorption in tissue is dominated by resonant absorption features, andno simple scaling can be assumed. For near-infrared light (˜0.8 μm),where absorption is relatively week, scattering is the dominantmechanism of attenuation. At longer wavelengths, such as 1.3 μm, 1.55 μmor 1.9 μm, scattering is minimal, and water absorption becomesincreasingly important.

[0164] The longitudinal resolution governed by the coherence length isinversely proportional to the optical bandwidth of the light source.

[0165] By the present invention the penetration depths may be increasedor even doubled due to the increased SNR depending on the opticalproperties of the medium.

[0166] The transversal resolution is essentially given by the well-knowndiffraction limit, i.e. the minimum focal spot, which is the resolvingpower. The diffraction limit is determined by the wavelength, theeffective aperture of the beam and the focal length of the lens as knownfrom the art.

[0167] The measurement speed, i.e. the time to perform a single a-scanand capture the interference signal, may be defined in different ways,and therefore a unique measure for this quantity cannot be given.However, increasing the scan speed implies increasing the electricalbandwidth of the detecting means and this may ultimately lead to anincrease of the receiver noise. As shown by the analysis above, theintroduction of the optical amplifier amplifying the reflected lightfrom the sample may be even more advantageous if the noise in thedetecting means increases. In other words, the optical amplifier may toa certain extent aid to overcome receiver noise.

[0168] Thus, due to the amplification system according to the presentinvention it is possible to conduct a faster scanning than withstate-of-the-art systems.

[0169] Transverse Scanning

[0170] The light path preferably includes a transverse scanningmechanism for scanning the probe beam within the sample, for example anactuator for moving the apparatus in a direction substantiallyperpendicular to the sample. Such a scanning mechanism can have amicro-machined scanning mirror. A longitudinal scanning mechanism canalso be provided to scan in a direction parallel to the probe beam.Scanning allows the apparatus to create images. Longitudinal scanning inthe direction of the probe beam axis, along with scanning in a directionperpendicular to the axis, provides the possibility of obtaining animage of a vertical cross section of the sample.

[0171] It is of course understood that although it is preferred to scanthe sample apparatus in relation to the sample, the sample may also bescanned with respect to a stationary sample probe or a combination ofthese.

[0172] Applications

[0173] The apparatus and method according to the present invention maybe used in any application normally applying OCT scanning as well newtechnical fields wherein the increased SNR allows the use of the presentapparatus. Thus, the apparatus may be used for so-called opticalbiopsies, wherein a segment of tissue, such as the skin, mucosa or anyother solid tissue is examined by OCT to diagnose any cellularabnormalities, such as cancer or cancer in situ. Furthermore, anymalignant growth may be detected by the present apparatus.

[0174] Due to the optical amplification conducted as discussed herein itis possible to increase the relative SNR, for example up to about 20 dB,such as about 17 dB, such as about 14 dB, drastically increasing thepenetration depth of the system. Thus, malignacies in the skin or mucosamay be detected directly by using the present invention. Furthermore,the apparatus may be coupled to catheters or the like to scan internalbody parts, such as the gastro-intestinal tract, a vessel or the heartor any body cavity. Also, the apparatus may be used for scanning duringa surgical operation.

[0175] Also, the present apparatus has improved the use of OCT inophthalmic application due to the increased penetration depth, such asin corneal topography measurements and as an aid in ophthalmic surgery,for example for focusing on the posterior intraocular lens capsule foruse in cataract surgery.

[0176] The present invention may also be applied in conventional OLCRapplications, such as detection or imaging of inhomogeneities in opticalwaveguides or devices, i.e. wherein the sample is an optical waveguideor an integrated optical device.

[0177] In another embodiment the sample may be a polymer or likestructure.

[0178] In yet another embodiment the sample may be silicon-basedintegrated circuit.

EXAMPLES

[0179] Here follows a comparison of a coherent optical FMCWreflectometry system according to prior art and two coherent opticalFMCW reflectometry systems according to the present inventionexemplifying the added benefit of introducing an optical amplifier. Theeffect of using a system where all undesired reflections has beenreduced as much as possible e.g. through coating of all surfaces, aswell as the effect of changing the splitting ratio on the splitter fromthe source, is demonstrated. The former case in which all surfaces arecoated is referred to as the coated case, whereas not coating thesurfaces is referred to as the uncoated case.

[0180] The system parameters used are

[0181]

I_(source)

=10 mW

[0182] Scan length of the light source =100 nm $\begin{matrix}{{\Delta \quad \gamma} = {\frac{1}{2{\pi\tau}_{c}} = {20\quad {MHz}}}} \\{{{Center}\quad {wavelength}} = {\frac{c}{v} = {1050\quad {nm}}}}\end{matrix}$

[0183] γ=2.72727·10¹⁸ Hz/s

[0184] B₀=v_(max)−v_(min)=27.2727 THz

[0185] N_(sp)=2

[0186] q=0.8

[0187] Receiver noise density =˜155 dBm/Hz

[0188] and the reflectivity profile of the sample arm is chosen to be$\begin{matrix}{{{r\left( \tau_{0} \right)} = {{r_{und}{\delta \left( {\tau_{0} - \tau_{und}} \right)}} + {\sum\limits_{i = 1}^{4}{r_{i}{\exp \left\lbrack {{- 2}{\mu\tau}_{i}} \right\rbrack}{\delta \left( {\tau_{0} - \tau_{i}} \right)}}} + {r_{b}{\exp \left\lbrack {{- 2}{\mu\tau}_{0}} \right\rbrack}}}},} & (24)\end{matrix}$

[0189] where $r_{und} = \left\{ \begin{matrix}0.04 & {{in}\quad {the}\quad {uncoated}\quad {case}} \\10^{- 5} & {{{in}\quad {the}\quad {coated}\quad {case}}\quad}\end{matrix} \right.$

[0190] r₁=0.001 and τ₁=0

[0191] r₂=0.001 and τ₁=c/(n·0.5 mm)

[0192] r₃=0.004 and τ₁=c/(n·1 mm)

[0193] r₄=0.004 and τ₁=c/(n·1.5 mm)

[0194] r_(b)=10⁻⁶ and μ=5 mm⁻¹

[0195] and the Signal-to-noise ratio is inspected for the measurement ofthe reflectivity r₃ and position τ₃. Here an attenuation coefficientμ_(t)=5 mm⁻¹, a probing depth z=2 mm, and a reflection coefficientwithin the sample of 0.4% have been chosen.

Example 1a

[0196] a) Choice of Reference System

[0197] First, a reference system against which to compare theperformance is decided. This system can be seen in FIG. 5. Through asimilar analysis as to the one used to derive Eq. (19), the SNR of thereference system may be found. By doing so, it is found that the optimumsplitting ratio toward the sample is approximately 35% and 65% towardsthe reference both for a system with and without coated surfaces. Asystem with this splitting ratio is chosen as reference system. This isshown in FIGS. 14 and 15 where r_(und) is in the uncoated and coatedcase, respectively. A system with this splitting ratio is chosen asreference system.

[0198] In the graphs in FIGS. 6-FIG. 11, relative SNR implies that thesystem under investigation is compared to the corresponding referencesystem, which experiences the same conditions in terms of reflectivity,receiver noise etc. The novel system in FIG. 2 is compared to thereference system.

[0199] b) Optimum Splitting Ratio in the Absence of Amplification

[0200] First, the splitting ratio x is investigated in the absence of anamplifier for both the coated and uncoated case. The SNR for this systemis found from Eq. (19) by letting G=1 and N_(sp)=0. FIGS. 6 and 7 showsthe SNR of the system shown in FIG. 2 as a function of the splitterratio x relative to the reference system FIG. 3 when r_(und) is in theuncoated and coated case, respectively.

[0201] From FIGS. 6 and 7 it is concluded that when an optical amplifieris not present in the system adjusting the splitting ratio away from50/50 is a disadvantage.

[0202] c) Effect of Amplification for a Constant Splitting Ratio

[0203] Next, the effect of the amplifier is investigated over a widerange of amplification factors and the splitting ratio is set to 50/50.FIG. 8 shows the increase in SNR due to the use of an optical amplifierin the novel system shown in FIG. 2 for the uncoated case and FIG. 9shows the increase in SNR in the coated case. It is noted that therelative SNR is less than unity for low amplification factors. This isdue to the amplifier having to compensate for the loss of optical signalpower due to the extra coupler in the system under investigation andadded amplifier noise.

[0204] d) Optimum Splitting Ratio for a Fixed Amplification

[0205] A conservative amplification factor of 100 (20 dB) is chosen andthe effect of choosing a different splitting ratio than 50/50 isinvestigated again. FIG. 10 and FIG. 11 shows the uncoated and thecoated case, respectively. These graphs demonstrate that in both casesit is an advantage to select a different splitting ratio then 50/50 fora system using optical amplification, and that the advantage of doingthis is highest in the coated case. In this case the SNR is imprivedwith about 9 dB.

[0206] e) Optical Circulators and Fixed Amplification

[0207] Another realization of the novel coherent optical FMCWreflectometry system is shown in FIG. 4, where the y-coupler in thesample part has been replaced with a so-called optical circulator knownfrom the art. Obviously, the signal light power is increased by a factorof four. Using the same parameters as above in d) for the coated case,the improvement in relative SNR for the realization in FIG. 4 is 31compared to 10 for the realization in FIG. 2 when both realizations arecompared to the same reference system.

[0208] Finally, in FIG. 12 the sensitivity of the relative SNR on thereceiver noise is demonstrated for the realization in FIG. 4. For a lowthermal noise the optical amplifier may be a disadvantage since thenoise is dominated by the noise added by the optical amplifier. As thethermal noise in the receiver is increased the optical amplifier becomesan increased advantage because the optical noise added is graduallymasked by the thermal noise. For high values of the thermal noise theadvantage of the optical amplifier is constant since the thermal noiseis the dominant noise term.

Example 1b

[0209] a) Choice of Reference System

[0210] The reference system chosen for this example is identical to thatof example 1a.

[0211] In the graphs in FIGS. 16-FIG. 21, relative SNR implies that thesystem under investigation is compared to the corresponding referencesystem, which experiences the same conditions in terms of reflectivity,receiver noise etc. The novel system in FIG. 13 is compared to thereference system. The SNR for the system shown in FIG. 13 is easilyfound by replacing Eq. 21 and Eq. 23 in the substitution into Eq. 19with

E _(s)={square root}{square root over ((1−x)x

I _(source)

)}  (25)

[0212] and $\begin{matrix}{{{\langle I_{sam}\rangle} = {\left( {1 - x} \right) \times {\langle I_{source}\rangle}{\int_{- \infty}^{\infty}{{r^{2}\left( \tau_{0} \right)}{\tau_{0}}}}}},} & (26)\end{matrix}$

[0213] respectively

[0214] c) Effect of Amplification for a Constant Splitting Ratio

[0215] Next, the effect of the amplifier is investigated over a widerange of amplification factors and the splitting ratio is set to 50/50.FIG. 16 shows the modest increase in SNR due to the use of the opticalamplifier in the uncoated case. This is due to the increase in amplifiernoise through mixing of the light due to the undesired reflectionr_(und) and the spontaneous noise power emitted by the amplifier. FIG.17 show the increase in SNR due to the use of the optical amplifier inthe coated case. For both cases it is noted that the relative SNR isless than unity for very low amplification factors. This is due to theamplifier having to compensate for the extra noise added by theamplifier.

[0216] d) Optimum Splitting Ratio for a Fixed Amplification

[0217] A conservative amplification factor of 100 (20 dB) is chosen andthe effect of choosing a different splitting ratio than 50/50 isinvestigated again. FIG. 18 and FIG. 19 shows the uncoated and coatedcase, respectively. These graphs demonstrate that in both cases it is anadvantage to select a different splitting ratio than 50/50 for a systemusing optical amplification although for chosen system parameters, theadvantage is only slight and the optimum splitting ratio is 45/55. It isseen that the relative increase in SNR by inclusion of an opticalamplifier is approximately a factor 25 higher in the coated casecompared to the uncoated case, and there is little advantage in theuncoated case.

[0218] The system realisations analyzed above should be consideredtypical examples. However, the advantage of introducing anoptical-amplifier is clearly pointed out. Firstly, it is demonstratedthat the optical amplifier may aid to overcome receiver noise leading toimproved system performance in terms increased SNR. The impact ofoptical amplification on an coherent optical FMCW reflectometry systemis highly dependent on the noise contribution from the receiving system,which comprises all components involved in obtaining an electricalsignal from the optical output e.g. electrical amplifiers, computer datacollection system etc. This sensitivity is illustrated through FIG. 12,where the system with optical amplifier and optical circulator (shown inFIG. 4) is compared to the reference system in the coated case.Secondly, an optimum splitting ratio different from 50/50 has beendemonstrated. Finally, adding an optical amplifier will be an increasedadvantage as the electrical bandwidth of the receiver is increased,which may lead to an increase of the receiver noise. In other words, theoptical amplifier may to a certain extent aid to overcome the increasein receiver noise. An increase in electrical detection bandwidth isnecessary when fast acquisition of measurement data desired e.g. forreal-time imaging.

1. An apparatus for optical coherence reflectometry comprising awavelength scanning laser source for providing a light signal splittingmeans for dividing said light signal into a first light field and asecond light field, means for directing the first light field to asample, and means for directing a first reflected light field from thesample, wherein an optical amplifier is inserted in the first reflectedlight field, said optical amplifier being different from the lightsource, and means for directing the amplified first reflected lightfield to a combining means, so that the amplified first reflected lightfield is directed to the combining means through another route than aroute through the splitting means for dividing the light signal, meansfor directing the second light field to the combining means, combiningmeans for receiving said amplified first reflected light field and saidsecond light field to generate a combined light signal, and at least onedetecting means for detecting the combined light signal and outputtingdetection signals.
 2. The apparatus according to claim 1, wherein thewavelength scanning laser source is an external-cavity frequency-tunedlaser.
 3. The apparatus according to claim 1 or 2, wherein the opticalamplifier is a semiconductor resonator, amplifier, resonant amplifier,fibre and/or Raman amplifier.
 4. The apparatus according to any of thepreceding claims, wherein the first light field is directed to thesample without being amplified.
 5. The apparatus according to any of thepreceding claims, wherein substantially all light energy from the firstreflected light field is directed to the combining means.
 6. Theapparatus according to any of the preceding claims, whereinsubstantially all light energy from the second light field is directedto the combining means.
 7. The apparatus according to any of thepreceding claims; wherein the optical path of the second light fieldcomprises a reflecting means, such as a mirror or a retroreflector. 8.The apparatus according to any of the preceding claims, comprising meansfor altering the optical length of the reference path with the purposeof inducing a frequency shift in the detected signal, such as a opticalmodulator, for an electro-optic modulator or a fibre stretcher.
 9. Theapparatus according to any of the preceding claims, wherein thesplitting means is bulk-optic, fibre optic or a hologram.
 10. Theapparatus according to any of the preceding claims, wherein thesplitting ratio of the splitting means is substantially 50%/50%.
 11. Theapparatus according to any of the preceding claims 1-9, wherein thesplitting ratio of the splitting means is changeable, so that from 1% to99% of the light energy from the light source is directed to the samplearm.
 12. The apparatus according to claim 11, wherein less than 50% ofthe light energy is directed to the sample.
 13. The apparatus accordingto any of the preceding claims, wherein two detecting means are arrangedto obtain a balanced detection signal.
 14. The apparatus according toany of the preceding claims, wherein at least one CCD camera is arrangedas a part of the detecting means to detect a part of the first reflectedlight field.
 15. The apparatus according to any of the preceding claims,wherein at least a part of the means for directing the first light fieldis an optical fibre.
 16. The apparatus according to claim 15, furthercomprising means for reducing non-sample reflection(s), such asfibre-end reflection(s).
 17. The apparatus according to claim 16,wherein the fibre-ends are anti-reflection coated.
 18. The apparatusaccording to claim 16 or 17, wherein the fibre-ends are cleaved at anangle.
 19. The apparatus according to claim 18, wherein the angle is atleast 5 degrees, such as at least 7 degrees.
 20. The apparatus accordingto any of the preceding claims, further comprising an actuator means formoving the apparatus in a direction substantially parallel to thesample.
 21. The apparatus according to any of the preceding claims,further comprising an actuator means for moving the apparatus in adirection substantially perpendicular to the sample.
 22. The apparatusaccording to any of the preceding claims, further comprising processingmeans for providing a result of the detection signals.
 23. The apparatusaccording to claim 22, further comprising a display device displayingthe result from the processed detection signals.
 24. A method forproviding a result of a sample comprising establishing a wavelengthscanning laser source for providing a light signal, splitting said lightsignal into a first light field and a second light field, directing thefirst light field to a sample, and the second light field to a referencepath, receiving the first reflected light field from the sample,optically amplifying the first reflected light field, receiving thesecond light field, combining said amplified first reflected light fieldand said second light field to generate a combined light signal,detecting the combined light signal obtaining detection signals, andprocessing the detection signals obtaining the result image of thesample.
 25. The method according to claim 24, wherein the sample is skinor mucosa.
 26. The method according to claim 24, wherein the sample isretina.
 27. The method according to claim 24, wherein the sample is avessel or heart.
 28. The method according to claim 24, applied during asurgical operation.
 29. The method according to claim 24, wherein thewavelength scanning laser source is an external-cavity frequency tunedlaser.
 30. The method according to claim 24, wherein the opticalamplifier is a semiconductor resonator, an amplifier, a resonantamplifier, fibre and/or Raman amplifier.
 31. The method according to anyof the preceding claims 24-30, wherein the first light field is directedto the sample without being amplified.
 32. The method according to anyof the preceding claims 24-31, wherein substantially all light energyfrom the first reflected light field is directed to the combining means.33. The method according to any of the preceding claims 24-32, whereinsubstantially all light energy from the second light field is directedto the combining means.
 34. The method according to any of the precedingclaims 24-33, wherein the optical path of the second light fieldcomprises a reflecting means, such as a mirror or a retroreflector. 35.The method according to any of the preceding claims 24-34, wherein thesplitting means is bulk-optic, fibre optic or a hologram.
 36. The methodaccording to any of the preceding claims 24-35, wherein the splittingratio of the splitting means is substantially 50%/50%.
 37. The methodaccording to any of the preceding claims 24-36, wherein the splittingratio of the splitting means is changeable, so that from 1% to 99% ofthe light energy from the light source is directed to the sample arm.38. The method according to claim 37, wherein less than 50% of the lightenergy is directed to the sample.
 39. The method according to any of thepreceding claims 24-38, wherein two detecting means are arranged toobtain a balanced detection signal.
 40. The method according to any ofthe preceding claims 24-39, wherein at least a part of the means fordirecting the first light field is an optical fibre.
 41. The methodaccording to claim 40, further comprising means for reducing non-samplereflection(s), such as fibre-end reflection(s).
 42. The method accordingto claim 41, wherein the fibre-ends are anti-reflection coated.
 43. Themethod according to claim 41 or 42, wherein the fibre-ends are cleavedat an angle.
 44. The method according to claim 43, wherein the angle isat least 5 degrees, such as at least 7 degrees.
 45. The method accordingto any of the preceding claims 24-44, further comprising an actuatormeans for moving the first light field in a direction substantiallyparallel to the sample.
 46. The method according to any of the precedingclaims 24-45, further comprising an actuator means for moving the firstlight field in a direction substantially perpendicular to the sample.47. The method according to claim 24, wherein the wavelength of thelight source is in the range from 500 nm to 2000 nm.
 48. The methodaccording to claim 25 or 27, wherein the wavelength of the light sourceis in the range from 1250 nm to 2000 nm.
 49. The method according toclaim 26, wherein the wavelength of the light source is in the rangefrom 600 nm to 1100 nm.